1. Field of the Invention
This invention relates to bipolar transistors, and more particularly, to a method for manufacture of an interdigitated collector structure for such transistors.
2. Prior Art
In the design of bipolar power transistors, three goals must be taken into consideration. It is desirable to have as high a breakdown voltage as possible while attaining minimum collector series resistance as well as minimum base-collector capacitance.
These three goals are difficult to attain in one transistor. For example, when the resistivity of the material between the base and the collector is low, the capacitance between the base and the collector is high. This requires a compromise to be struck between low resistivity and low capacitance. Also, providing for a low collector series resistance tends to result in a lower junction breakdown voltage.
One prior art method for raising the junction breakdown voltage while retaining a low collector series resistance is to implant guard rings in the substrate surrounding the base. These guard rings have the function of raising the breakdown voltage around the edges of the base, the location where breakdown is most likely. However, the use of guard rings requires extra processing as well as using up extra valuable space.
Another prior art method to achieve all three goals without the use of guard rings is to form low resistance pedestals up from the collector into a high resistivity epitaxial layer between the collector and the base and directly beneath each emitter. This has been accomplished by heavily doping discrete regions in the collector prior to the formation of the epitaxial layer. After the deposition of the epitaxial layer, the device is heated causing the dopant therein to out-diffuse from the collector into the epitaxial layer, thus doping the upwardly adjacent regions in the epitaxial layer to form pedestals on top of the collector. The emitters were then aligned to be located directly above each of these pedestals.
This prior art method has a number of drawbacks. First, the double diffusion, first into the collector and then the out-diffusion into the epitaxial layer, results in imprecisions in the formation in the pedestals. One such imprecision is in the level of doping of the pedestals. It has been found that the method described results in a high gradient of doping from the bottom of the pedestal adjacent to the collector to the top of the pedestal. This results in a difficulty in accurately manufacturing a device with known base-collector resistance. It is also difficult to precisely predetermine the shapes of the pedestals. Irregularities such as rounding occur. The result of these problems is that the design of the transistor is limited to these designs which can tolerate such irregularities. In addition, the operations required to form the pedestals using this method are cumbersome and costly.